Update README.md

Author: JorjMcKie

README.md

@@ -30,9 +30,14 @@ The platform tag for Mac OSX is `macosx_10_6_intel`.
 
 The platform tag for Linux is `manylinux1_x86_64`, which makes these wheels usable on Debian, Ubuntu and most other variations.
 
-On other operating systems you need to generate PyMuPDF yourself. Before you can do this, you must download and generate MuPDF. This process depends very much on your system. For most platforms, the MuPDF source contains prepared procedures for achieving this.
+On other operating systems you need to generate PyMuPDF yourself. And of course you can choose to do so for a wheel-supported platform, too. Before you can do this, you must download and generate MuPDF. This process depends very much on your system. For most platforms, the MuPDF source contains prepared procedures for achieving this.
 
-Be sure to download the official MuPDF source release from [here](https://mupdf.com/downloads). Do **not use** MuPDF's [GitHub repo](https://github.com/ArtifexSoftware/mupdf). It contains their current **development source**, which is **not compatible** with this PyMuPDF version most of the time.
+* Be sure to download the official MuPDF source release from [here](https://mupdf.com/downloads). Do **not use** MuPDF's [GitHub repo](https://github.com/ArtifexSoftware/mupdf). It contains their current **development source**, which is **not compatible** with this PyMuPDF version most of the time.
+
+* The repo's `fitz` folder contains a few files whose names start with an underscore. These files contain configuration data and hotfixes and each must be copy-renamed to its correct target location of the downloaded MuPDF source **before you generate MuPDF**. Currently, these files are:
+  - fitz configuration file `_mupdf_config.h` copy-replace to `mupdf/include/fitz/config.h`.
+  - fitz error module `_error.c`, copy-replace to `mupdf/source/fitz/error.c`.
+  - PDF device module `_pdf-device.c` copy-replace `mupdf/source/pdf/pdf-device.c`.
 
 Once this is done, adjust directories in ``setup.py`` and run ``python setup.py install``.